Spring modulation with fast load-transient response for a voltage regulator

ABSTRACT

A spring modulation is proposed to regulate the output voltage of a voltage regulator. The spring modulation comprises a differential amplifier to generate a pair of voltage signal and current signal varied with the difference between a reference signal and a feedback signal related to the output voltage, and a PWM generator to generate a PWM signal in response to the pair of voltage signal and current signal to regulate the output voltage, in such a manner that, in load transient, the greater the difference between the feedback signal and reference signal is, the greater the on-time duty-cycle of the PWM signal is.

FIELD OF THE INVENTION

The present invention is related generally to a voltage regulator andmore particularly to a spring modulation for a voltage regulator.

BACKGROUND OF THE INVENTION

Voltage regulator has been applied in various electronic products toprovide stable supply voltages for load on the voltage regulator. FIG. 1schematically shows a typical voltage regulator 10 operated withconstant on-time duty-cycle, which comprises a pair of high side MOStransistor 12 and low side MOS transistor 14 coupled between inputvoltage V_(IN) and ground GND, inductor 18 coupled between the phasenode 16 between the MOS transistors 12 and 14 and output V_(OUT) of thevoltage regulator 10, output capacitor C_(OUT) coupled between theoutput V_(OUT) and ground GND and having equivalent series resistance(ESR) R_(ESR), resistors R₁ and R₂ coupled in series between the outputV_(OUT) and ground GND to divide the output voltage V_(OUT) to generatefeedback signal V_(FB), error amplifier 20 to compare the feedbacksignal V_(FB) with reference voltage V_(REF) to generate error signal PMfor constant on-time circuit 22 to generate a pulse width modulation(PWM) signal PWM accordingly, to switch the MOS transistors 12 and 14 bydriver 24 to thereby produce an output current I_(OUT) flowing throughthe inductor 18 to charge the output capacitor C_(OUT) to produce theoutput voltage V_(OUT) supplied to load 26.

In the voltage regulator 10, the PWM signal has a constant on-timeduty-cycle. However, the switching frequency of the PWM signal isvariable. FIG. 2 shows waveforms of the output voltage V_(OUT) and PWMsignal in the voltage regulator 10, in which waveform 50 represents theoutput voltage V_(OUT) and waveform 52 represents the PWM signal. Asshown at time T in FIG. 2, when the load 26 changes from light to heavy,the output voltage V_(OUT) drops down instantly, causing a greaterdifference between the feedback signal V_(FB) and reference voltageV_(REF), which reflects on the error signal PM, having the constanton-time circuit 22 to increase the switching frequency of the PWMsignal. As a result, the high side MOS transistor 12 turns on morefrequently, and thereby the output voltage V_(OUT) recovers back to theoriginal level gradually.

In response to load transient, it is the switching frequency, but notthe on-time duty-cycle, of the PWM signal being varied in the voltageregulator 10, and this constant on-time duty-cycle operation is easilyto cause the output voltage V_(OUT) to overshoot, as shown by the dottedcurve 54 in FIG. 2. Therefore, the system becomes not stable, and theoutput capacitor C_(OUT) is required to have larger capacitance,resulting in higher cost and poor conversion efficiency.

SUMMARY OF THE INVENTION

The present invention is directed to a spring modulation for a voltageregulator.

More particularly, the present invention is directed to a modulationwith fast load-transient response for a voltage regulator.

In a spring modulation for a voltage regulator, according to the presentinvention, a differential amplifier generates a pair of voltage signaland current signal varied with the difference between a reference signaland a feedback signal related to the output voltage of the voltageregulator, and a PWM generator generates a PWM signal in response to thepair of voltage signal and current signal to regulate the outputvoltage. In load transient, the greater the difference between thefeedback signal and reference signal is, the greater the on-timeduty-cycle of the PWM signal is. Therefore, as the feedback signalapproaches to the reference signal, the on-time duty-cycle of the PWMsignal decreases gradually.

By using the spring modulation according to the present invention formodulating the on-time duty-cycle of a voltage regulator, fastload-transient response is achieved, the output capacitors and therebythe cost therefor could be decreased, the output voltage of the voltageregulator will not overshoot even the load-transient response is fast,and the efficiency of the voltage regulator increases.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, features and advantages of the presentinvention will become apparent to those skilled in the art uponconsideration of the following description of the preferred embodimentsof the present invention taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 schematically shows a conventional voltage regulator usingconstant on-time duty-cycle;

FIG. 2 depicts waveforms of the output voltage V_(OUT) and PWM signal inthe voltage regulator shown in FIG. 1;

FIG. 3 shows a voltage regulator employing a spring modulation accordingto the present invention;

FIG. 4 shows an embodiment for the differential amplifier of the voltageregulator shown in FIG. 3;

FIG. 5 depicts waveforms of the output voltage V_(OUT) and PWM signal inthe voltage regulator shown in FIG. 3; and

FIG. 6 is a comparison of the waveforms shown in FIGS. 2 and 5.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 shows a voltage regulator 100 employing a spring modulationaccording to the present invention. In the voltage regulator 100, lowside MOS transistor 102 and high side MOS transistor 104 are coupled inseries between input voltage V_(IN) and ground GND, spring modulationcircuit 101 switches the MOS transistors 102 and 104 with driver 120 toproduce output current I_(OUT) flowing through inductor 106 to chargeoutput capacitor C_(OUT) to produce output voltage V_(OUT) supplied toload 118, the output capacitor C_(OUT) has an equivalent seriesresistance R_(ESR), resistors R₁ and R₂ are coupled in series betweenthe output V_(OUT) and ground GND to divide the output voltage V_(OUT)to produce feedback signal V_(FB), differential amplifier 108 generatesdifferential voltage ΔV and differential current ΔI from the differencebetween the feedback signal V_(FB) and reference signal V_(REF), thedifferential voltage ΔV is coupled to the set input S of RS latch 114 inPWM generator 109, the differential current ΔI is coupled to the inputof hysteretic comparator 116 in the PWM generator 109, the PWM generator109 further comprises resistor 110 and capacitor 112 coupled in seriesbetween the output Q of the SR latch 114 and ground GND, the nodebetween the resistor 110 and capacitor 112 has voltage V_(ΔI) thereonand is coupled to the input of the hysteretic comparator 116, the outputof the hysteretic comparator116 is coupled to the reset input R of theRS latch 114, and the output Q of the SR latch 114 generates PWM signalfor the driver 120 to switch the MOS transistors 102 and 104. Theon-time duty-cycle of the PWM signal varies with the difference betweenthe feedback signal V_(FB) and reference signal V_(REF). In particular,the greater the difference is, the greater the on-time duty-cycle is,and vice versa. The spring modulation circuit 101 functions like aspring connected between the desired level and real level of the outputvoltage V_(OUT) to maintain the voltage regulator 100 producing stableoutput voltage V_(OUT) by modulating the on-time duty-cycle of the PWMsignal in response to the difference between the desired level and reallevel of the output voltage V_(OUT).

FIG. 4 shows an embodiment for the differential amplifier 108 of thevoltage regulator 100, which comprises PMOS transistors 150, 152, 154,156 and 158, NMOS transistors 160, 162, 164, 166, 168, 170 and 172, andcurrent source 174. The PMOS transistors 150 and 152 constitute adifferential input pair to be coupled with the feedback signal V_(FB)and reference signal V_(REF), the current source 174 provides biascurrent, the MOS transistors 154, 156, 160, 162, 164 and 166 form abuffer stage, and the differential voltage ΔV is derived from the drainof the PMOS transistor 156. The MOS transistors 154, 158, 160, 162, 164and 168 form another buffer stage, and the differential current ΔI isgenerated by the current mirror composed of the NOMS transistors 170 and172. The outputs ΔV and ΔI of this differential amplifier 108 are bothproportional to the difference between the feedback signal V_(FB) andreference signal V_(REF).

FIG. 5 depicts waveforms of the output voltage V_(OUT) and PWM signal inthe voltage regulator 100, in which waveform 200 represents the outputvoltage V_(OUT) and waveform 202 represents the PWM signal. Referring toFIGS. 3 and 5, when the load 118 changes from light to heavy, the outputvoltage V_(OUT) drops down instantly, and the feedback signal V_(FB)also drops down since it is proportional to the output voltage V_(OUT),resulting in greater difference between the feedback signal V_(FB) andreference signal V_(REF). Therefore, the differential amplifier producesgreater differential voltage ΔV and differential current ΔI, such thatthe on-time duty-cycle of the PWM signal increases eventually. As theoutput voltage V_(OUT) increases, the difference between the signalsV_(FB) and V_(REF) decreases gradually, and the on-time duty-cycle ofthe PWM signal decreases accordingly. Until the output voltage V_(OUT)recovers back to the original level, the on-time duty-cycle of the PWMsignal recovers to its original value. In load transient, the waveform202 of the PWM signal behaves like a recovering spring. For comparison,the waveforms of FIGS. 2 and 5 are shown together in FIG. 6. When loadtransient occurs at time T, the voltage regulator 100 employing thespring modulation according to the present invention responds morequickly to recover the output voltage V_(OUT) thereof by increasing theon-time duty cycle, while the conventional voltage regulator 10 usingconstant on-time duty cycle takes longer time period.

In this embodiment, the difference between the feedback signal V_(FB)and reference signal V_(REF) is used for modulating the on-timeduty-cycle of the PWM signal, and therefore the on-time duty-cycle ofthe PWM signal decreases gradually in load transient, and thereby theoutput voltage V_(OUT) will not overshoot. Since the spring modulationaccording to the present invention has fast load-transient response, theoutput capacitor is allowed to have smaller capacitance, and thus theefficiency of the voltage regulator is improved. Further, for themanufacture of the voltage regulator, the number of the output capacitoris reduced owing to the smaller capacitance required, and thus the costof the voltage regulator is lower.

While the present invention has been described in conjunction withpreferred embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and scopethereof as set forth in the appended claims.

1. For a voltage regulator modulated by a PWM signal of an on-timeduty-cycle to produce an output voltage, a spring modulation methodcomprising the steps of: modulating the on-time duty-cycle in responseto a difference between a reference signal and a feedback signal variedwith the output voltage.
 2. The method of claim 1, wherein the on-timeduty-cycle gradually decreases in a load transient.
 3. The method ofclaim 1, wherein the step of modulating the on-time duty-cycle comprisesthe steps of: generating a voltage signal and a current signal inresponse to the difference; and modulating the on-time duty-cycle basedon the voltage signal and the current signal.
 4. For a voltage regulatormodulated by a PWM signal of an on-time duty-cycle to produce an outputvoltage, a spring modulation circuit comprising: apparatus formodulating the on-time duty-cycle in response to a difference between areference signal and a feedback signal varied with the output voltage.5. The circuit of claim 4, wherein the on-time duty-cycle graduallydecreases in a load transient.
 6. The circuit of claim 4, wherein theapparatus comprises: a differential amplifier receiving the feedbacksignal and reference signal for generating a voltage signal and acurrent signal varied with the difference; and a PWM generator inresponse to the voltage signal and current signal for generating the PWMsignal.
 7. The circuit of claim 6, wherein the PWM generator comprises:an RS latch having a set input coupled with the voltage signal; ahysteretic comparator having an output coupled to a reset input of theRS latch; and a charger coupled with the current signal for generating acharged voltage coupled to an input of the hysteretic comparator.